STUTTGART, Germany -- Tipped by U.S. authorities, German police arrested a Turkish man and his American fiancee for allegedly plotting to attack U.S. military bases in Heidelberg on the anniversary of the Sept. 11 terror attacks, German authorities said Friday.

Authorities on Thursday found 287 pounds of chemicals and five pipe bombs along with a picture of Osama bin Laden in the couple's apartment near Heidelberg, home to the U.S. Army Europe headquarters.

"We suspect that they intended to mount a bomb attack against military installations and the city of Heidelberg," the chief law enforcement officer for Baden-Wuerttemberg state, Thomas Schaeuble, told reporters.

Authorities were looking into whether the couple was acting alone or were part of a network. However, federal authorities investigating the Hamburg terror cell that participated in the Sept. 11 attacks on Washington and New York have left the new case in the hands of state officials, giving an indication that the plans appeared isolated.

The 25-year-old Turkish man and his 23-year-old fiancee both indicated a hatred for Jews, Schaeuble said. Neither were identified, but U.S. officials in Washington said the woman had dual U.S. and German citizenship.

Their occupations would have given both the access necessary to carry out the planned attack, Schaeuble indicated. The man worked in a chemical warehouse in nearby Karlsruhe. The woman worked in a supermarket at a U.S. installation in Heidelberg, where some 16,000 American soldiers, family members and civilian workers are stationed.

There were no signs of heightened alert at the Campbell Barracks in Heidelberg Friday evening. Joggers ran past the fenced-in headquarters and children played outside at military housing across the street.

Earlier this week, federal Interior Minister Otto Schily said German authorities had no information on planned attacks around the anniversary of the Sept. 11 attacks and that 500 tips indicating potential threats since last year's attacks were not well-founded.

U.S. counterterrorism officials in Washington said Friday they have detected a mild increase in the terrorist "chatter" in the run-up to Sept. 11, but don't believe it is a sign of a major impending attack.

The "chatter" -- intelligence from various sources that monitor suspected terrorists' communications -- fluctuates often and was far greater around the July 4 holiday, one official said.

While officials are concerned al-Qaida may try an attack on Sept. 11, they note the group has never marked anniversaries or holidays, unlike other terrorist organizations.